1. Field of the Invention
The present invention relates to a solid image pickup device and a method for producing the same, and in particular a solid image pickup device of mono-layered electrode structure and a producing method thereof.
2. Description of the Related Art
CCD solid image pickup devices used to such as area sensors have an electric charge transfer electrode for transferring signal charges from a photoelectric transferring section. A plurality of electric charge transfer electrodes is disposed on electric charge transferring paths located on a semi-conductor substrate, and each of the electric charges is arranged next to each other and driven in succession.
In the solid image pickup device, the number of imaging pixels is increasing. With increasing of the number of the imaging pixels, high-speed transfer of the signal charge, that is, drive by high-speed pulse of the charge transfer electrode is required. Therefore, the charge transfer electrode is demanded to have low resistance. As a method for producing the low resistance, it has been proposed to produce the charge transfer electrodes with two-layered structure of a silicon based conductive material such as a polycrystalline silicon and a metallic silicide.
On the other hand, an area of the photoelectric transferring section tends to become narrower with increasing the number of the imaging pixels. For concentrating much light at a narrow area, it is important to further lower height of surroundings of the photoelectric transferring section, such as charge transfer electrode producing section, with respect to the surface of the photoelectric transferring section. Therefore, the charge transfer electrode having a so-called mono-layered structure has been proposed, not overlapping the charge transfer electrodes one another. With the charge transfer electrode having the mono-layered structure, a light shielding property of a shielding film on the transfer electrode parts is improved more effectively.
However, in case the charge transfer electrode of the mono-layered structure is driven at high-speed pulse, a distance (gap) between the charge transfer electrodes arranged next to each other must be formed narrowly (0.1 xcexcm or less). For providing pattern sizes as this degree (0.1 xcexcm or less), a stepper of high cost is required as in a manner of using an EB direct drawing method. Moreover, even if the electrode pattern can be obtained, it is extremely difficult to fill the insulating film in a fine area between electrodes, probably causing pressure proof to be weakened, and practically insufficient.
Further, in case of that the charge transfer electrodes has the two-layered structure of the silicon based conductive material and the metallic silicide, it is difficult to oxidize metals of high melting points or metallic compounds of high melting points such as tungsten or tungsten suicide. Even if oxidation can be effected, electric pressure proof of an obtained insulating film is not sufficient. Therefore, it is impossible to form insulating films practically available caused by generating oxidation between the electrodes.
It is also proposed a two-layered structure charge transfer electrode, which has composed of the silicon based conductive material and the metallic silicide (see JP-A-2000-196060), instead of a mono-layered structure charge transfer electrode. But, only the surface of the silicon based conductive material is arranged with the metallic suicide, and the low resistance is not yet sufficient. Moreover, the distance between the electrodes of the charge transfer electrode is around 0.25 to 0.50 xcexcm.
The invention has been built in view of the above mentioned-circumstances, and accordingly it is an object of the invention to offer such a solid image pickup device having high electric pressure proof between the charge transfer electrodes of mono-layered structure and enabling to drive at high speed with low consumption electric power, and a method of making such a solid image pickup device of simple structure and high reliability.
The present invention provides a solid image pickup device formed with a plurality of charge transfer electrodes located on an insulating film of a surface of a semi-conductor substrate. The solid image pickup device has inter-electrode insulating films formed on the insulating film between the charge transfer electrodes arranged next to one another. In the solid image pickup device, the charge transfer electrodes include adhesion is films so formed as to cover side walls of the inter-electrode insulating films and the insulating film, and conductive films containing metals formed in ranges surrounded with the adhesion films.
With such a structure, the inter-electrode insulating films of fine width size can be formed. And the inter-electrode insulating films are covered closely and desirably, leaving no spaces, with the adhesion film which is so formed as to cover the side walls of the inter-electrode insulating films. Moreover, the conductive film containing the metal of low resistance is formed in the upper layer of the conductive film. For the reasons stated above, it is possible to produce electrodes having lower resistance. Further, the surface of the conductive film can be made flat. Therefore, in case a wiring structure is formed thereon, the pattern can be efficiently formed.
The present invention provides a distance of 0.1 xcexcm or less between the charge transfer electrodes formed with the inter-electrode insulating films of the solid image pickup device. If the distance between the charge transfer electrodes is 0.1 xcexcm or less, it is extremely difficult to fill the insulating film between the electrodes. However, since the conductive film containing the adhesion film and metals is so formed as to cover the inter-electrode insulating films, it is possible to reduce the distance between the charge transfer electrodes to be 0.1 xcexcm or less. Accordingly, it is possible to provide the solid image pickup device of low resistance and high reliability. Therefore, the charge transfer electrodes can be driven at high pulse.
The adhesion film of the solid image pickup device of the invention is a silicon based conductive film. The polycrystalline silicon film or the amorphous silicon film can be utilized as the adhesion film. The silicon based conductive film is easy to form a film excellent in shielding stepwise differences in level depending on, for example, a pressure reducing CVD. Moreover, the silicon based conductive film also has a good adhesion with metallic layers such as tungsten. If the polycrystalline silicon film is applied as an adhesion film, it is possible to provide a further low resistance by doping.
The conductive film of the solid image pickup device of the invention is a metallic silicide film. With this structure, it is possible to provide a still further low resistance.
The conductive film of the solid image pickup device of the invention includes tungsten. With this structure, the low resistance can be provided, and at the same time, a light shielding function can be provided owing to tungsten. Therefore, it is possible to omit a shielding film conventionally demanded and obtain the solid image pickup device at low cost and of high reliability.
The inter-electrode insulating film of the solid image pickup device of the invention is a film formed by thermal oxidation of a silicon based material. As the silicon based material, a non-doped silicon may be employed.
With this structure, the silicon based material such as the polycrystalline silicon or the amorphous silicon is subjected to the thermal oxidation, thereby enabling to provide the close insulating film. In addition, using the non-doped silicon, impurities as phosphorus are prevented from contribution to conductivity, so that insulation can be made secure between electrodes. Accordingly, since the inter-electrode insulating film of high quality is formed, pressure proof can be increased.
The present invention also provides a method for producing a solid image pickup device formed with a plurality of charge transfer electrodes on a gate insulating film of a surface of a semi-conductor substrate. The method includes the steps of: forming inter-electrode insulating films for forming the gate insulating film pattern serving as the inter-electrode insulating film on the insulating film; forming an adhesion film so as to cover side walls of the inter-electrode insulating films and the gate insulating film; forming a conductive film on the upper layer of the adhesion film such that the surface of the conductive film is made flat; and performing etch-backs on the conductive film and the adhesion film until a top face of the inter-electrode insulating film is exposed.
According to this method, once passing through a photolithographic process, it is possible to form the patterns of the insulating film becoming the inter-electrode insulating film of desired reliability.
Furthermore, the step of forming the inter-electrode insulating film in the producing method of the invention includes the steps of: forming the inter-electrode insulating film on the insulating film; forming a resist pattern having a width larger in size than an objective distance between electrodes on the surface of the insulating film for forming the inter-electrode insulating film; reducing a pattern width in size by carrying out an isotropic treatment on the resist pattern; and patterning for etching the insulating film for forming the inter-electrode insulating film as a mask of the resist pattern.
According to this method, a resist pattern is formed at a resolution limit and is subjected to a further size reduction treatment, whereby the width is made fine. Therefore fine resist patterns may be easily produced, and the inter-electrode insulating film may be formed with desired reliability.
Moreover, the step of forming the inter-electrode insulating film in the producing method of the invention includes the steps of: forming a silicon based conductive film on the insulating film; forming the resist pattern having the width larger in size than the objective distance between the electrodes on the surface of the silicon based conductive film; reducing the pattern width by carrying out the isotropic treatment on the resist pattern; patterning for etching the silicon based conductive film as the mask of the resist pattern; and forming an oxidized silicon film by oxidizing the pattern of the silicon based conductive film produced by the patterning step.
According to this method, the resist pattern is formed at a resolution limit and is subjected to a further size reduction treatment, whereby the width is made fine. Therefore, fine resist patterns may be produced, and the silicon based conductive film is passed through the patterning, followed by oxidation, so that the fine patterns of the oxidized silicon film of high quality can be easily obtained.
The size reducing step of the producing method of the invention depends on an ashing step. Following this method, the resist pattern can be made fine only through the thermal treatment.
The above mentioned etch-back step of the producing method of the invention depends on a chemical machining process (CMP). According to this method, it is possible to form the solid image pickup device of good surface flatness by the CMP.
The conductive film forming step of the producing method of the invention includes a step of forming a tungsten film by a pressure reducing CVD, and at the same time, the light shielding effect may be also brought about.
In accordance with the invention, since the wiring material of low resistance is applied as the charge transfer electrodes, a device can be reduced in height size. Therefore, processing margins are broadened in the photolithographic process or the etching process. For the reasons stated above, the solid image pickup devices may be produced at high yield without using a semi-conductor producing apparatus as expensive steppers. Furthermore, since the insulating film of high quality is applied as the inter-electrode insulating films, electric pressure proof can be improved with high yield. In addition, the insulating material is not needed to be buried in ranges between the electrodes of fine width, and the electric pressure proof can be avoided from going down, and the yield can be increased.